Slugs don’t have outer shells because they evolved from snails and lost their external shells over time. They no longer needed these shells because they found ways to survive without having a shell to hide in for protection or using a shell to store their internal organs. Slugs stopped growing outer shells because:.
You could be wondering “Why do snails have shells and slugs don t?”
The shell both provides them protection and makes it so they can’t go as many places as the slug can. Slugs may not have that calcium carbonate shell to protect their bodies and internal organs like a snail, but they can easily tuck themselves into all sorts of areas, even those too tight for a snail’s shell to get into.
One of the next things we asked ourselves was; what is a slug without a shell called?
The answer is an slug (Arionvulgaris). [RN] Snails without a shell are generally referred to as slugs. But the reduction of an exterior shell not only happens among land snails. In the sea you might find the colourful nudibranchs – them as well survive without a shell.
Slugs have evolved from snails many times (in different families/superfamilies of snails), There are a number of “semi-slugs” that have external shells too small to withdraw into. Slugs lose the protection of the shell, but avoid carrying the weight, and the considerable expenditure of energy needed to make it.
One more inquiry we ran across in our research was “Why do snails and slugs hang out on top of logs?”.
The shelled critter may hang out on the upper side of the stone or log, but the shell keeps them from getting underneath into that glorious dampness. Snails and slugs are loved and hated by many for different reasons.
Why do shells vary in size and shape?
Varying the size of the opening relative to the overall size of the shell (by increasing order, from left to right; shells like the first three, where sucessive whorls do not touch, appear to be significantly less common than others, perhaps because they have lower mechanical rigidity; they occur mostly as internal rather than external shells):.
This of course begs the question “How does a mollusk enlarge its shell?”
The mollusk does not enlarge its shell in a uniform way: it only adds material in one of the edges of the shell (the open or “growth” ending) and makes it in such a way that the new shell is always an exact model, to scale, of the smaller shell.
As the animal grows inside the shell, its home must get bigger, and so they grow their shells layer upon layer, creating ‘growth-bands’, or growth increments, within the shell. Over time, these close-to- identical bands form patterns.
A s mollusks develop in the sea, their mantle tissue absorbs salt and chemicals. They secrete calcium carbonate, which hardens on the outside of their bodies, creating a hard shell. The mollusk continues to take in salt and chemicals from the sea and secrete calcium carbonate, which makes its shell grow even bigger.
Why do shells have patterns?
The formation of shells can also create patterns. As the animal grows inside the shell, its home must get bigger, and so they grow their shells layer upon layer, creating ‘growth-bands’, or growth increments, within the shell. Over time, these close-to- identical bands form patterns.
This of course begs the question “Why are shells different colours?”
Most shell colourings are caused by pigments incorporated into the shell or it’s outer ‘skin’ coating. Usually the pigments are incorporated in a regular pattern, such as stripes or spirals of colour. In other shells, it’s not so regular, with spotty or splotchy colourings. The formation of shells can also create patterns.
Why do sea shells have patterns?
While sea shells serve as protective homes for their fragile inhabitants, shell patterns have a less obvious purpose. Visually striking markings can be used to attract a mate or provide camouflage, but elaborately decorated clams and mussels can spend their entire lives unseen, hidden under a rock or buried in sand.
Why do seashells grow from the bottom up?
Thus, seashells grow from the bottom up, or by adding material at the margins. Since their exoskeleton is not shed, molluscan shells must enlarge to accommodate body growth. Damaged seashells, on the other hand, use secretions of proteins and calcium from the mantle cells underneath the shell for repair.
The effects of varying some of the growth parameters of seashells may be observed in the following pictures. They show that, with the same model, every type of seashell shape may be produced.